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Chapter 7 Earthing and Shielding
If you wish to avoid the disadvantages associated with creating an earth loop with two-sided shields,
it is recommended you connect one side of the shield through a high impedance.
For longer lengths of shielded cables, such as if a sensor must be added to a control panel, a
potential difference between both end points must not be ignored.
However, such shield conductors are relatively expensive and also require more time in working with
them. Another possibility would be to place an additional voltage equalizing cable between the
measurement location and the control panel. The shield can then be hooked up on both sides.
A high-impedance earth connection is also another option. In the control panel, the shield is then
connected to the earth potential, and the shield has a high-impedance connection to earth at the
measurement location via a gas discharge tube. This solves the problem of a potential transfer and
50-Hz humming.
For non-isolated measurement locations, two gas discharge tubes must be installed. One connects
the shield to earth, and the other connects it to the non-isolated measurement location. This method
prevents a galvanic coupling between the measurement circuit and the earthed measurement
location.
GFK-2958E
RSTi-EP User Manual
331
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